It seems to me a lawyer with 40 years of experience should be able to answer a layman's question as to what other case in which someone charged with multiple homicides in the same jurisdiction was given separate trials for each.
To be blunt, it seems that way to you because you are a know-it-all on almost every topic you know nothing about. Probably 97% of all cases, including murders, do not result in reported decisions. A reported decision is published only if the case is appealed and the appellate court believes its opinion is significant enough to publish. I have no idea, and am certainly not going to attempt to research, whether there are reported decisions involving the issue of whether a trial involving multiple murders should have been severed. Probably there are, but surely not many. The Texas Penal Code pretty much tells us the answer: One trial is permissible if two crimes are part of the same "episode," but the defense may object that trying the crimes together would be unfairly prejudicial. Hence, the determination is made on a case-by-case basis. It is highly unlikely that two murders separated in time, location and circumstance, requiring entirely different evidence and witnesses, would be tried together. The prejudice to Oswald would be exactly why an LN zealot like you would
want them tried together - if he killed Tippit, ladies and gentlemen of the jury, he must've killed JFK. It is highly unlikely that the prosecution would even want them tried together because this would be handing the defense an almost guaranteed grounds for appeal. In the case of the Tippit murder, I believe the defense would have a solid threshold argument that this was not even the same episode as the JFK murder.
Lawyers go to law school to learn to "think like a lawyer." Believe me, your nuance-free, black-and-white view of every issue would've put you at the bottom of the class.